Poly (ether ether ketone) (PEEK) is a robust engineering plastic with good thermal and chemical stability. The quaternized ammonium (QA) functionalized PEEK provides anion transfer capacity for anion-exchange membrane fuel cells (AEMFC), because the introduction of hydrophilic QA groups or QA-based side chains. QA-PEEK membrane has been a promising solution for AEMFC in recent years thanks to the improvement of synthesis method^1-3 and the abundance of grafting groups^4,5. The divergence of hydrophilicity between backbones and side chains of QA-PEEK could result in microphase separation due to the spontaneous aggregation. In our previous work⁵, we reported the ions conductivity of two QA groups functionalized PEEK (GQ-PEEK, Fig.(a)) is higher than that of one QA groups (SQ-PEEK, Fig.(a)) under same grafting degree (GD). However, the mechanism of the improved conductivity still needs more investigation because various parameters could affect the ions conductivity, for example, ion exchange capacity (IEC), micro-phase separation, as well as effective ions diffusion coefficient.

Generally, the micro-structure of the membrane and ions transportation process could be effectively investigated by molecular dynamics simulations^6,7. In the present work, we carried out molecular dynamics simulation to investigate the microphase separation and transportation process (H₂O and OH^-) of SQ-PEEK and GQ-PEEK respectively. The calculated structure factor (Fig.(b)) reveals the possible microphase separation in both membrane. The cluster size of GQ-PEEK (~3.69 nm) is slightly larger than that of SQ-PEEK (~3.24 nm), which is in consistent with the TEM results (Fig.(c)). Meanwhile, the self-diffusion coefficients (Fig.(d)) of both H₂O and OH^- in GQ-PEEK have no obvious difference than those in SQ-PEEK. Therefore, the increased conductivity of GQ-PEEK should attribute to the increased IEC in comparison to SQ-PEEK. This work provides a deeper understanding of anion-exchange membrane for fuel cells.

Reference

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(3) Yan, X.; He, G.; Gu, S.; Wu, X.; Du, L.; Zhang, H. Journal of Membrane Science 2011, 375, 204.

(4) Si, J.; Wang, H.; Lu, S.; Xu, X.; Peng, S.; Xiang, Y. Journal of Materials Chemistry A 2017, 5, 4003.

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(7) Lyu, K.; Peng, Y.; Xiao, L.; Lu, J.; Zhuang, L. Journal of Energy Chemistry 2018.